Glass tube processing method, apparatus and glass tube

ABSTRACT

In a glass processing method according to the invention, in the case of performing chemical vapor deposition or diameter shrinkage of a substrate glass tube G by relatively moving a heating furnace  20  comprising a heating element  21  for annularly enclosing the circumference of the substrate glass tube in a longitudinal direction of the substrate glass tube G with respect to the substrate glass tube G in which an outer diameter is 30 mm or more and a wall thickness is 3 mm or more and is less than 15 mm and an ovality of the outer diameter is 1.0% or less using a glass processing apparatus  1 , a temperature of at least one of the heating element  21  and the substrate glass tube G is measured and the amount of heat generation of the heating element  21  is adjusted based on the measured temperature.

BACKGROUND OF THE INVENTION

The present invention relates to a glass tube processing method andapparatus for performing chemical vapor deposition or diameter shrinkageof a substrate glass tube by relatively moving the substrate glass tubeand a heating source in a longitudinal direction of the substrate glasstube, and further a glass tube in which chemical vapor deposition ordiameter shrinkage is performed.

In a step of manufacturing an optical fiber preform, an chemical vapordeposition step (see, for example, Non-patent Reference 1) of forming aglass layer inside a glass tube or a step of shrinking a glass tube to adesired diameter is performed. In these steps, a glass tube issequentially heated in a longitudinal direction of the glass tube by aheating source provided outside the glass tube.

For example, in an chemical vapor deposition step called an chemicalvapor deposition CVD method, glass raw material gas for generating glassfine particles (SiO₂) is introduced into the inside of a substrate glasstube used as a substrate of chemical vapor deposition and a heatingsource provided outside the substrate glass tube is transversely movedalong a longitudinal direction of the substrate glass tube and thesubstrate glass tube is heated. By heating the substrate glass tubethus, the glass raw material gas put into the inside of the substrateglass tube is oxidized and the glass fine particles are generated. Then,the glass fine particles are deposited on an inner surface of thesubstrate glass tube in the downstream side of a stream of the glass rawmaterial gas. Thereafter, the deposited glass fine particles are heatedby traverses of the heating source and become transparent and a glasslayer is sequentially formed.

Such a chemical vapor deposition step is repeatedly performed and untila wall thickness of the substrate glass tube reaches a desiredthickness, a plurality of glass layers are formed and a glass tubeforming an intermediate of the optical fiber preform can be formed.

Further, in a diameter shrinkage step, as a previous step ofimplementing collapse (wherein a glass pipe of which a hollow portion isfilled in and becomes a glass rod) of a substrate glass tube by, forexample, a collapse method or a rod-in collapse method, the substrateglass tube is heated along a longitudinal direction of the substrateglass tube and is softened and a diameter of the substrate glass tube isshrunk by surface tension similar to that of the collapse method.

[Non-patent Reference 1]

“Optical Fiber Communications International Edition 1991”, McGraw-HillBook Co., p. 66-67

By the way, as a heating source use in such glass processing, anoxyhydrogen burner is generally used. When the oxyhydrogen burner isused, since its flame rises upwardly, normally, a substrate glass tubearranged in a horizontal direction is heated by applying the flame fromthe lower side thereof with being rotated about the axis thereof. Inthat case, the flame is not applied directly to the upper side of thesubstrate glass tube, so that it is difficult to obtain uniformtemperature distribution over a circumferential direction of thesubstrate glass tube and circumferential bias occurs in viscosity of thesubstrate glass tube.

As a result of that, there were cases that a shape of the glass tube isdeformed after the processing. Also, there were cases where the softenedsubstrate glass tube is shrinked locally due to wind pressure caused bythe flame.

When the glass tube is deformed by the processing thus and its sectionalshape becomes oval, trouble about the optical fiber preform is caused.

For example, when collapse (wherein a glass pipe of which a hollowportion is filled in and becomes a glass rod) of a glass tube formed inan chemical vapor deposition step is implemented by a collapse method toform a glass rod and a core portion of the optical fiber preform isformed, a core of an optical fiber obtained from its preform alsobecomes oval. Then, transmission performances are degraded due to, forexample, occurrence of polarization mode dispersion.

Incidentally, such ovality of the glass tube was enhanced remarkablywhen a diameter of the substrate glass tube is large and a thickness ofthe substrate glass tube is thin.

SUMMARY OF THE INVENTION

An object of the invention is to provide a glass processing method andapparatus capable of suppressing ovality of a glass tube in the case ofperforming processing such as chemical vapor deposition or diametershrinkage by heating a substrate glass tube, and a glass tube processedthereby.

A glass processing method according to the invention capable ofachieving the object is characterized in that in the case of performingchemical vapor deposition or diameter shrinkage of a substrate glasstube by relatively moving a heating furnace comprising a heating elementfor annularly enclosing the circumference of the substrate glass tube ina longitudinal direction of the substrate glass tube with respect to thesubstrate glass tube in which an outer diameter is 30 mm or more and awall thickness is 3 mm or more and is less than 15 mm and an ovality ofthe outer diameter is 1.0% or less, a temperature of at least one of theheating element and the substrate glass tube is measured and the amountof heat generation of the heating element is adjusted based on themeasured temperature.

Incidentally, an ovality of the outer diameter can be defined by thefollowing formula (I) when the maximum value is a and the minimum valueis b and the average value is c among outer diameters on one arbitrarycircumference of a substrate glass tube.

{(a−b)/c}×100  (1)

Also, in the glass tube processing method according to the invention,the amount of heat generation is preferably adjusted at least one timealong with the relative movement with respect to a region with 100 mm orlonger of a longitudinal direction of the substrate glass tube.

Also, in the glass tube processing method according to the invention, atemperature of an external surface of the substrate glass tube ispreferably measured through a void part provided in the heating elementusing a temperature measuring device provided outside the heatingelement.

Also, in the glass tube processing method according to the invention, inthe case of performing the chemical vapor deposition or the diametershrinkage, a distance along a longitudinal direction of the substrateglass tube from a place of the maximum temperature of the glass tube toa place in which a temperature lowers by 30° C. than the maximumtemperature is preferably set at 20 mm or longer.

Also, in the glass tube processing method according to the invention, inthe case of performing the chemical vapor deposition or the diametershrinkage, a speed relative movement of the heating element ispreferably set at 10 mm/min or higher.

Also, in the glass tube processing method according to the invention, inthe case of performing the chemical vapor deposition or the diametershrinkage, a difference between the maximum temperature and the minimumtemperature in a circumferential direction of the substrate glass tubeis preferably set at 200° C. or lower.

Also, in the glass tube processing method according to the invention, inthe case of performing the chemical vapor deposition or the diametershrinkage, the substrate glass tube is preferably rotated about thecenter axis of the substrate glass tube at speeds of 10 rpm or higherand 150 rpm or lower.

Also, in the glass tube processing method according to the invention, inthe case of performing the chemical vapor deposition or the diametershrinkage, with respect to a region with 100 mm or longer of alongitudinal direction of the substrate glass tube, an outer diameter ofthe substrate glass tube is measured along with the relative movementand a pressure of the inside of the substrate glass tube is preferablyadjusted at least one time based on the measured outer diameter.

Also, in the glass tube processing method according to the invention,the outer diameter is preferably measured using an outer diametermeasuring device selected from the group of a laser light type monitor,a CCD camera and an X-ray camera.

Also, in the glass tube processing method according to the invention,the outer diameter is preferably measured through a void part providedin the heating element using an outer diameter measuring device providedoutside the heating element.

Also, in the glass tube processing method according to the invention, inthe case of performing the chemical vapor deposition or the diametershrinkage, a pressure difference between the inside and the outside ofthe substrate glass tube is preferably adjusted at 1500 Pa or lower.

Also, in the glass tube processing method according to the invention, aratio ID/Od between an outer diameter Od of the substrate glass tube andan inside diameter ID of the heating element is preferably set in therange of 1.1 to 5.0.

Also, a glass tube according to the invention is characterized in thatthe glass tube is processed using a glass tube processing method of theinvention and an ovality of an outer diameter is 0.5% or less.

Also, a glass tube processing apparatus according to the invention ischaracterized by comprising a heating furnace comprising a heatingelement for annularly enclosing the circumference of a substrate glasstube which is a heated object, a gas supply part for supplying gas tothe inside of the substrate glass tube, a gas exhaust part forexhausting gas from the inside of the substrate glass tube, movementmeans for relatively moving the substrate glass tube and heating furnacein a longitudinal direction of the substrate glass tube, a temperaturemeasuring device for measuring a temperature of at least one of theheating element and the substrate glass tube, and heat generation amountadjusting means for adjusting the amount of heat generation of theheating element.

Also, in the glass tube processing apparatus according to the invention,the glass tube processing apparatus preferably comprises an outerdiameter measuring device for measuring an outer diameter of thesubstrate glass tube, and pressure adjusting means for adjusting apressure of the inside of the substrate glass tube.

Also, in the glass tube processing apparatus according to the invention,the heating element preferably has a void part passing through the innercircumference side and the outer circumference side of the heatingelement.

Also, in the glass tube processing apparatus according to the invention,the void part is a hole and a plurality of holes are formed and arepreferably arranged so as not to align in any of a circumferentialdirection or a longitudinal direction of the heating element.

Also, in the glass tube processing apparatus according to the invention,the heating furnace is an induction furnace comprising an induction coilin the circumference of the heating element, and the void part is ahole, and an opening area of the inner circumference side per one holeof the hole arranged inside the induction coil is 1000 mm² or less, andthe total opening area of the inner circumference side of the holearranged inside the induction coil is preferably 50% or less an area ofthe inner circumference side of the heating element located inside theinduction coil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the whole schematic view of a glass tube processing apparatuscapable of implementing a glass tube processing method according to theinvention.

FIG. 2 is a schematic view showing a heating furnace shown in FIG. 1.

FIG. 3 is a schematic view showing a configuration for controlling apressure of the inside of a substrate glass tube.

FIG. 4 is a perspective view showing an example of another heatingelement.

FIG. 5 is a schematic view of the case of performing diameter shrinkagein the glass tube processing method according to the invention.

FIG. 6 is a graph showing a result of an example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Examples of embodiments of a glass tube processing method, a glass tubeprocessing apparatus and a glass tube according to the invention will bedescribed below with reference to the drawings.

A glass tube processing apparatus capable of implementing a glass tubeprocessing method according to the invention is shown in FIG. 1.

A glass tube processing apparatus 1 shown in FIG. 1 is means forgenerating glass fine particles inside a substrate glass tube by theso-called chemical vapor deposition method (chemical vapor depositionCVD method) and depositing the glass fine particles inside the substrateglass tube and forming a glass film.

The glass tube processing apparatus 1 has a pedestal 12 on which supportparts 11 are stood in the vicinity of both ends. The support parts 11respectively have rotatable chucks 13 and these chucks 13 respectivelygrasp the ends of a substrate glass tube G and the substrate glass tubeG is supported horizontally.

A heating furnace 20 for heating the substrate glass tube G is providedbetween the two support parts 11. A heating furnace comprising a heatingelement for annularly enclosing the substrate glass tube G can be usedas this heating furnace 20 and, for example, an induction furnace or aresistance heating furnace can be used. In the present embodiment, thecase of using the induction furnace will be described.

The heating furnace 20 is mounted with respect to a support rail(movement means) 14 provided between the support parts 11 on thepedestal 12, and can be moved along a longitudinal direction of thesupport rail 14. The support rail 14 is arranged in parallel with thecenter axis of the substrate glass tube G grasped by the chucks 13, andthe heating furnace 20 moves in parallel with the center axis of thesubstrate glass tube G.

Also, a gas supply tube (gas supply part) 15 is connected to one side(left side in the drawing) of the support parts 11, and a buffer tank 16and a gas exhaust tube (gas exhaust part) 17 are connected to the otherside (right side in the drawing) of the support parts 11. These gassupply tube 15, buffer tank 16 and gas exhaust tube 17 form a flow pathof gas continuous with space of the inside of the substrate glass tubeG.

Also, gas introduction means (not shown) for introducing gas into spaceof the inside of the substrate glass tube G is connected to the gassupply tube 15. The gas introduction means is constructed so thatsilicon tetrachloride (SiCl₄), oxygen (O₂), helium (He), germaniumtetrachloride (GeCl₄), etc. can be introduced as single gas or properlymixed gas.

The heating furnace 20 shown in FIG. 1 will be described. As shown inFIG. 2, the heating furnace 20 of the embodiment is a furnace of a highfrequency induction heating method, and a heating element 21 generatesheat by passing an AC current through an induction coil 23. The heatingelement 21 has a cylindrical shape annularly enclosing the circumferenceof the substrate glass tube G, and the material is graphite or zircon.This heating element 21 generates heat to a temperature higher than orequal to a softening point of glass to heat and soften the substrateglass tube G. Incidentally, a softening point is about 1700° C., whenmaterial of the substrate glass tube G is glass with high purity formedby a VAD method etc.

The induction coil 23 is arranged so as to heat an axial center portionof the heating element 21, and the number of turns of the induction coilis set properly.

Also, an insulator 22 is provided between the heating element 21 and theinduction coil 23.

Also, the heating furnace 20 includes a temperature measuring device 25for measuring a temperature of the heating element 21 and a temperaturemeasuring device 24 for measuring a temperature of the substrate glasstube G. The temperature measuring device 25 may be either a contact typeor a noncontact type, and the temperature measuring device 24 is aradiation thermometer of a noncontact type. A temperature of a regionheated by the heating element 21 in the substrate glass tube G ismeasured by the temperature measuring device 24, and preferably, atemperature of an external surface of a portion located inside theinduction coil 23 can be measured. For that purpose, void parts passingthrough the inner circumference side and outer circumference are formedas holes 21 a, 22 a respectively in the insulator 22 and the heatingelement 21 arranged inside the induction coil 23.

By such a configuration, a temperature of an external surface of aheated region of the substrate glass tube G can be directly measured bythe temperature measuring device 24 arranged outside the induction coil23 through the holes 21 a, 22 a.

Also, the temperature measuring devices 24, 25 are connected to acurrent control part 26 and can send respective measured values to thecurrent control part 26. The current control part 26 is means foradjusting magnitude of current passed through the induction coil 23based on temperatures measured by the temperature measuring devices 24,25. The amount of heat generation of the heating element 21 can beadjusted by adjusting the magnitude of current passed through theinduction coil 23. That is, the current control part 26 functions asheat generation amount adjusting means of the heating element 21. Also,the amount of heat generation may be adjusted by adjusting magnitude ofvoltage applied to the induction coil 23.

Also, a plurality of temperature measuring devices 24 for measuring atemperature of the substrate glass tube G may be provided in order tomeasure temperatures of plural places of the substrate glass tube G.

Incidentally, in the invention, a temperature of at least any one of theheating element 21 and the substrate glass tube G could be measured.

Also, the heating furnace 20 comprises an outer diameter measuringdevice 27 capable of measuring an outer diameter of the substrate glasstube G. Any of a laser light type monitor, a CCD camera and an X-raycamera could be used as the outer diameter measuring device 27. Also,the outer diameter measuring device 27 can preferably measure an outerdiameter of a place immediately after being heated or a place heated inthe substrate glass tube G. In the embodiment, the outer diametermeasuring device 27 is arranged outside the induction coil 23, and anouter diameter of a place heated can be measured through the holes 21 a,22 a of the insulator 22 and the heating element 21 separately formedfor temperature measurement described above. Also, the outer diametermeasuring device 27 is connected to a flow rate control part 28 and cansend a measured value to the flow rate control part 28. The flow ratecontrol part 28 is means for adjusting a pressure of the inside of thesubstrate glass tube G based on an outer diameter value measured by theouter diameter measuring device 27.

Next, a configuration for adjusting a pressure of the inside of thesubstrate glass tube G will be described with reference to a schematicview shown in FIG. 3.

As shown in FIG. 3, gas is supplied from one and side (left side in thedrawing) of the inside of the substrate glass tube G and the gas isexhausted from the other end side (right side in the drawing). Thebuffer tank 16 is provided between the substrate glass tube G and thegas exhaust tube 17 for exhausting the gas, and the gas is stagnated inthis buffer tank 16 before exhausting. A gas tube 31 is connected to thebuffer tank 16, and gas of a flow rate adjusted by a flow rate adjustingdevice (MFC) 30 is supplied to the inside of the buffer tank 16properly. Depending on the amount of gas supplied from the gas tube 31,a pressure of the inside of the buffer tank 16 changes and accordingly,a pressure of the inside of the substrate glass tube G also changes.

Also, a pressure gauge 29 for measuring a pressure of the inside of thebuffer tank 16 is mounted in the buffer tank 16. Also, the pressuregauge 29 is connected to the flow rate control part 28 and a measuredvalue is sent to this flow rate control part 28.

By such a configuration, based on an outer diameter value measured bythe outer diameter measuring device 27 and a pressure value of theinside of the buffer tank 16 measured by the pressure gauge 29, the flowrate control part 28 controls the flow rate adjusting device 30 toadjust a pressure of the inside of the buffer tank 16. By adjusting thepressure of the inside of the buffer tank 16, a pressure of the insideof the substrate glass tube G is adjusted and an outer diameter of thesubstrate glass tube G softened can be adjusted.

Also, by reason of the fact that the holes 21 a are formed in theheating element 21, there is a possibility that the temperature of aregion where the substrate glass tube G facing the holes 21 a becomeslower as compared with that of the circumference. Therefore, as shown inFIG. 2, a plurality of holes 21 a formed in the heating element 21 arepreferably arranged so as not to align in any of the samecircumferential direction or the same longitudinal direction in theheating element 21.

Further, by setting an opening area of the inner circumference side perone hole of the holes 21 a arranged inside the induction coil 23 at 1000mm² or less, an influence on heating by reason of the fact that theholes 21 a are formed can be reduced to maintain a uniform heatingcondition. Also, in order to maintain high heating efficiency by theheating element 21, it is desirable to reduce the total opening area ofthe inner circumference sides of all the holes 21 a arranged inside theinduction coil 23. For example, the total opening area of the innercircumference sides of the holes 21 a with respect to an area of theinner circumference side of the heating element 21 located inside theinduction coil 23 could be set at 50% or less.

Also, the heating element 21 described above has a cylindrical shape inwhich the holes 21 a are formed as a void part, but a shape of the heatgeneration body may be other forms as long as the circumference of thesubstrate glass tube is annularly enclosed.

For example, as another heating element, it may be a form with acylindrical shape as a whole in a state of meandering in a longitudinaldirection (right and left directions in the drawing) of the substrateglass tube G as shown in FIG. 4. In this form, a meandering gap is aslit-shaped void part 36 which is formed along the longitudinaldirection of the substrate glass tube G and passes through the innercircumference side and the outer circumference side of the heatingelement. Then, an outer diameter or a temperature of the substrate glasstube G can be measured through this void part 36.

Also, as shown in FIG. 4, electrodes are connected to both sides ofplaces discontinuous in a circumferential direction and heat of aheating element 35 can be generated by an electric resistance method. Inthat case, the induction coil as shown in FIG. 2 is not required.

Also, as another heating element, it may be a heating element in whichit is divided into two or more cylindrical shapes and its gap is formedas a void part.

Incidentally, in the invention, in order to measure a temperature or anouter diameter of the substrate glass tube from the radial outside ofthe heating element, a void part is not necessarily formed as a physicalspace. For example, any material through which sensor light (infraredrays, etc.) of the temperature measuring device or the outer diametermeasuring device can pass may be provided.

Next, a method for performing chemical vapor deposition in a substrateglass tube G using the glass tube processing apparatus 1 shown in FIGS.1 to 3 will be described.

Incidentally, in a substrate glass tube G used in the invention, anouter diameter is 30 mm or more and a wall thickness is 3 mm or more andis less than 15 mm and an ovality of the outer diameter is 1.0% or less.Preferably, an inner diameter could be 24 mm or more. In the case ofusing such a substrate glass tube with a large diameter and a thinthickness, an oval suppression effect of the invention can be obtainedremarkably.

Also, as a length of the substrate glass tube G, a substrate glass tubewith a length of, for example, about 600 mm can be used preferably.

Further, a ratio ID/Od between an outer diameter Od of the substrateglass tube and an inside diameter ID of the heating element ispreferably set in the range of 1.1 to 5.0 by properly setting an outerdiameter of the substrate glass tube G used. By such setting, a gap L3between the heating element 21 and the substrate glass tube G becomesnarrow and the substrate glass tube G can be heated efficiently andcircularity of a glass tube obtained by processing can be advanced.Also, space between the heating element 21 and the substrate glass tubeG becomes small and variations in external pressure applied to thesubstrate glass tube G can be decreased.

In the case of performing chemical vapor deposition, glass raw materialgas including silicon tetrachloride and oxygen is first introduced intothe inside of a substrate glass tube G by gas introduction means. Inorder to adjust a partial pressure of the silicon tetrachloride in theglass raw material gas, helium may be included in the glass raw materialgas. Also, the partial pressure of the silicon tetrachloride can beadjusted by the amount of oxygen.

The substrate glass tube G is rotated about the center axis of thesubstrate glass tube while the gas is properly introduced into theinside of the substrate glass tube G thus. A rotational speed is set at,for example, 10 rpm or higher and 150 rpm or lower. By setting therotational speed at 10 rpm or higher, a difference between temperaturesof a circumferential direction of the substrate glass tube G can bedecreased. For example, a difference between the maximum temperature andthe minimum temperature of a circumferential direction in a heatedregion is easily set at 200° C. or lower. As a result of this, adifference between viscosities of the circumferential direction of thesubstrate glass tube G can be decreased to prevent ovality. Further,preferably, a difference between the maximum temperature and the minimumtemperature of the circumferential direction in the heated region couldbe set at 50° C. or lower.

Also, by setting the rotational speed at 150 rpm or lower, occurrence ofwhirling of the substrate glass tube due to excessive centrifugal forcecan be suppressed.

Next, the heating element 21 generates heat by passing a current throughthe induction coil 23 so that a surface temperature of the outside ofthe substrate glass tube G reaches a desired temperature in the rangeof, for example, about 1900° C. to 2100° C.

Then, the heating furnace 20 is transversely moved from one end side ofthe substrate glass tube G toward the other end side (that is, along alongitudinal direction). At this time, a start position of the traverseis set at the side of the gas supply tube 15 to which glass rawmaterialgas is supplied. Incidentally, a traverse speed is set at 10 mm/min orhigher. By setting such a traverse speed, as shown in FIG. 2, a distanceL2 from a place T1 in which the maximum temperature of the substrateglass tube G is reached to a place T2 in which a temperature lowers by30° C. than the maximum temperature can be increased, and a rate ofchange in viscosity in the longitudinal direction of the substrate glasstube G can be decreased. Therefore, variations in an outer diameter inthe longitudinal direction of the substrate glass tube G can besuppressed.

Also, the distance L2 varies depending on a distance L1 at which theinduction coil 23 is provided, so that the distance L2 is easilyincreased by increasing the distance L1 of the induction coil 23previously. Incidentally, the distance L2 could be, for example, 20 mmor longer.

Further, in the embodiment, a temperature of an external surface of theheated substrate glass tube G is measured through the holes 21 a, 22 aby the temperature measuring device 24, a current of the induction coil23 is controlled by the current control part 26, and the amount of heatgeneration of the heating element 21 is adjusted so that the measuredvalue becomes a desired value. As a result of this, a temperature of thesubstrate glass tube G is controlled so as to become constant in alongitudinal direction and a shape of the substrate glass tube G can bemade uniform in the longitudinal direction.

Besides, a relation between a temperature of the heating element 21 anda temperature of the substrate glass tube G is previously examined andthe amount of heat generation of the heating element 21 may be adjustedso that the temperature of the substrate glass tube G becomes a desiredvalue based on the temperature of the heating element 21 measured by thetemperature measuring device 24.

Also, when the amount of heat generation of the heating element 21 isadjusted with using both of measured values of a temperature of theheating element 21 and a temperature of the substrate glass tube G, thetemperature of the substrate glass tube G can be controlled with higheraccuracy.

Incidentally, the amount of heat generation of this heating element 21could be adjusted continuously with respect to a region with 100 mm orlonger of a longitudinal direction of the substrate glass tube G duringat least one traverse.

Also, an outer diameter of the substrate glass tube G heated is measuredthrough the holes 21 a, 22 a by the outer diameter measuring device 27and based on an outer diameter value measured by the outer diametermeasuring device 27 and a pressure value of the inside of the buffertank 16 measured by the pressure gauge 29, the flow rate adjustingdevice 30 is controlled by the flow rate control part 28 so that themeasured value becomes a desired value. As a result of this, a pressureof the inside of the substrate glass tube G is adjusted along with apressure of the inside of the buffer tank 16 and an outer diameter ofthe substrate glass tube G softened is adjusted and can be maintained ata desired value. Therefore, a shape of the substrate glass tube G can bemade uniform in the longitudinal direction.

Also, in the case of adjusting a pressure, a pressure difference betweenthe inside and the outside of the substrate glass tube G could beadjusted so as to be at 1500 Pa or lower. By maintaining the pressuredifference between the inside and the outside of the substrate glasstube G at 1500 Pa or lower, sudden deformation of the substrate glasstube G can be prevented to suppress ovality.

Incidentally, this pressure could be adjusted continuously with respectto a region with 100 mm or longer of a longitudinal direction of thesubstrate glass tube G during at least one traverse.

As shown in FIG. 2, when the heating furnace 20 is transversely moved ina longitudinal direction of the substrate glass tube G with glass rawmaterial gas introduced, silicon tetrachloride is oxidized in the insideof the substrate glass tube G in a heated region, and glass fineparticles (called soot) G1 which are silica (SiO₂) are generated. Then,by thermophoresis, the glass fine particles G1 are deposited on theinside of the substrate glass tube in the downstream side of a stream ofthe glass raw material gas (called sooting). Then, a porous glass fineparticle deposit G2 is formed in a portion in which the glass fineparticles G1 are deposited and also, the deposit is heated by traversesof the heating furnace 20 and becomes transparent and a glass layer G3is sequentially formed.

After the glass layer G3 is deposited and the heating furnace 20 istransversely moved to the other end side (the side of the gas exhausttube 17) of the substrate glass tube G, a temperature of the heatingfurnace 20 is decreased to a temperature at which the glass fineparticles G1 are not generated in the substrate glass tube G (forexample, a temperature at which a surface temperature of the substrateglass tube G becomes about 500° C.). Then, the heating furnace 20 whosetemperature is decreased is returned by transversely moving the heatingfurnace 20 to the side of the gas supply tube 15 in which the sooting isstarted.

Further, reciprocating movement by the traverse described above isrepeated plural times and the glass layer G3 with a desired thickness isformed. As a result of this, a desired glass tube forming anintermediate of an optical fiber preform can be formed. Incidentally,the glass layer G3 with an adjusted refractive index can be formed byincluding gas such as germanium tetrachloride for refractive indexadjustment in gas supplied to the inside of the substrate glass tube G.

Also, in the glass tube processing method according to the invention,diameter shrinkage of a substrate glass tube can also be performed byperforming control similar to that of the chemical vapor deposition ofthe substrate glass tube described above.

As shown in FIG. 5, in the case of performing diameter shrinkage of asubstrate glass tube G, a glass tube with a small oval rate can beobtained by performing the diameter shrinkage while performing similartemperature control or pressure control using the glass tube processingapparatus 1 (see FIGS. 1 to 3). Incidentally, inert gas such as nitrogenor argon could be used as gas supplied to the inside of the substrateglass tube G. Also, the diameter shrinkage can be performed easily bysetting a pressure of the inside of the substrate glass tube G at anegative pressure with respect to the outside of the substrate glasstube G, and the oval rate can be reduced further by setting the pressureof the inside of the substrate glass tube G at a positive pressuresomewhat with respect to the outside of the substrate glass tube G.

According to the glass tube processing method and the glass tubeprocessing apparatus described above, ovality of a glass tube obtainedcan be suppressed effectively to obtain the glass tube in which anovality of an outer diameter is 0.5% or less.

Example

Next, an example according to the invention will be described.

Chemical vapor deposition with a substrate glass tube G was performedwhile performing the temperature control or pressure control describedabove using the glass tube processing apparatus 1 described above. Asubstrate glass tube in which an outer diameter was 35 mm and an insidediameter was 27 mm and an ovality was 0.3% was used as the substrateglass tube G. Also, a traverse speed of the heating furnace 20 at thetime of the chemical vapor deposition was 100 mm/min and a rotationalspeed of the substrate glass tube G was set at 40 rpm. Also, a pressureof the inside of the substrate glass tube G at the time of the chemicalvapor deposition was set at a pressure which was high by 80 Pa withrespect to an atmospheric pressure of the outside. Further, the maximumtemperature of an external surface of the substrate glass tube G heatedwas 2100° C.

Further, for comparison with a result of the present example, chemicalvapor deposition with the substrate glass tube G was similarly performedusing an oxyhydrogen burner as a heating source.

Then, outer diameters of respective glass tubes obtained in the case ofusing the oxyhydrogen burner and the case of using the heating furnace20 as the heating source were measured along a circumferential directionof any longitudinal places. A graph of this result is shown in FIG. 6.

As shown in FIG. 6, in the example according to the invention in whichthe chemical vapor deposition was performed using the heating furnace20, variations in the outer diameter were small and an ovality was about0.2%. On the contrary, in a comparative example in which the chemicalvapor deposition was performed using the oxyhydrogen burner, variationsin the outer diameter were large and an ovality became larger than 0.5%.

In accordance with the glass tube processing method and the glass tubeprocessing apparatus according to the invention thus, it was recognizedthat a glass tube with a small oval rate of the outer diameter could beobtained.

According to the invention, a glass processing method and apparatuscapable of suppressing ovality of a glass tube in the case of performingprocessing such as chemical vapor deposition or diameter shrinkage byheating a substrate glass tube and a glass tube processed thereby can beprovided.

1-18. (canceled)
 19. A glass tube processing apparatus comprising: aheating furnace including a heating element for annularly enclosing thecircumference of a substrate glass tube which is a heated object, theheating element having a void part passing through the innercircumference side and the outer circumference side of the heatingelement, a gas supply part for supplying gas to the inside of thesubstrate glass tube, a gas exhaust part for exhausting gas from theinside of the substrate glass tube, movement means for relatively movingthe substrate glass tube and the heating furnace in a longitudinaldirection of the substrate glass tube, a temperature measuring devicefor measuring a temperature of the substrate glass tube through the voidpart, and heat generation amount adjusting means for adjusting theamount of heat generation of the heating element.
 20. The glass tubeprocessing apparatus as claimed in claim 19, further comprising: anouter diameter measuring device for measuring an outer diameter of thesubstrate glass tube, and pressure adjusting means for adjusting apressure of the inside of the substrate glass tube.
 21. The glass tubeprocessing apparatus as claimed in claim 19, wherein the void part is aplurality of holes which are formed and arranged not to align in any ofthe same circumferential direction of the same longitudinal direction ofthe heating element.
 22. The glass tube processing apparatus as claimedin claim 19, wherein the heating furnace is an induction furnacecomprising an induction coil in the circumference of the heatingelement, the void part is a hole, an opening area of the innercircumference side per one hole of the hole arranged inside theinduction coil is 1000 mm² or less, and the total opening area of theinner circumference side of the hole arranged inside the induction, coilis 50% or less an area of the inner circumference side of the heatingelement located inside the induction coil.
 23. A glass tube processingapparatus comprising: a heating furnace including a heating elementannularly enclosing the circumference of a substrate glass tube which isa heated object, the heating element having a void part passing throughthe inner circumference side and the outer circumference side of theheating element, movement means for relatively moving the substrateglass tube and the heating furnace in a longitudinal direction of thesubstrate glass tube, a temperature measuring device which is providedoutside the heating element and is for measuring a temperature of anexternal surface of the substrate glass tube through the void part, andheat generation amount adjusting means for adjusting the amount of heatgeneration of the heating element.
 24. The glass tube processingapparatus as claimed in claim 23, further comprising: an outer diametermeasuring device for measuring an outer diameter of the substrate glasstube, and pressure adjusting means for adjusting a pressure of theinside of the substrate glass tube.
 25. The glass tube processingapparatus as claimed in claim 23, wherein the void part is a pluralityof holes which are formed and arranged not to align in any of the samecircumferential direction or the same longitudinal direction of theheating element.
 26. The glass tube processing apparatus as claimed inclaim 23, wherein the heating furnace is an induction furnace comprisingan induction coil in the circumference of the heating element, the voidpart is a hole, and an opening area of the inner circumference side perone hole of the hole arranged inside the induction coil is 1000 mm² orless.
 27. The glass tube processing apparatus as claimed in claim 23,wherein the heating furnace is an induction furnace comprising aninduction coil in the circumference of the heating element, the voidpart is a hole, and the total opening area of the inner circumferenceside of the hole arranged inside the induction coil is 50% or less anarea of the inner circumference side of the heating element locatedinside the induction coil.
 28. The glass tube processing apparatus asclaimed in claim 23, wherein the heating furnace is an induction furnacecomprising an induction coil in the circumference of the heatingelement, the void part is a plurality of holes which are formed andarranged not to align in any of the same circumferential direction orthe same longitudinal direction of the heating element, an opening areaof the inner circumference side per one hole of the holes arrangedinside the induction coil is 1000 mm² or less, and the total openingarea of the inner circumference side of the hole arranged inside theinduction coil is 50% or less an area of the inner circumference side ofthe heating element located inside the induction coil.